I don't see a downside. You will be using Migration Assistant, connecting the MacBook either in FireWire Target Disk Mode, or over a network.
The MacBook will act as the data source and this process does not change the source data (just copies it). So even in the worse case, you can decide to do it over again.
Also, depending on how your iMac "is a mess," you might want to use do one of the following two options first, before resorting to a complete +Erase and Install+.
(1) Just create a new admin user account on the iMac. You use System Preferences
Accounts pane to do this
http://docs.info.apple.com/article.html?path=Mac/10.5/en/8235.html
See if your iMac works properly in the new account. If so, you can copy over just your user data from the old account, or you can use Migration Assistant to migrate data off the MacBook Pro to the new account.
(2) Do a reinstall of Leopard, but use the +Archive and Install+ option in Installer. This will give you a fresh Leopard system, but your data and user settings should remain intact. If you use this approach, you should make a backup of your user data on the iMac, just in case something goes wrong.